The present invention is related to a method and apparatus for generating a geometric model, and particularly to a method and apparatus for generating a geometric model which is suitable for a CAD (Computer Aided Design) system for machine design.
The main object of a CAD system is to construct a graphic shape of a design objective in a computer. This shape expression is called a "geometric model." In the conventional CAD system, a geometric model is generated mainly by set operations of a solid. That is, first the geometry of each basic solid is determined in a local coordinate system, then a positioning is made in a space by an operation such as movement/rotation, and finally set operations such as sum, difference and product are performed between them.
In accordance with this method, if a step is cut in part of a block for instance, the shape to be cut out should be determined first in conformity with the block which is a base. Namely, it is necessary to calculate the shape of the step while referencing the parameters (such as distance between the planes) of the shape to the base. Apart from the intention of a designer to cut out stepwise, specification of the geometric data in detail is essential. This is a large burden to the designer. In addition, even if the size of the part to be cut out is not important, but the size of the part remaining is important, the size must be converted to the geometric data of the step and specified. That is, size specification different from the designer's intention may be required. Furthermore, if the values of the specified shape and size are inconsistent, it is often difficult to grasp and solve its cause.
In contradistinction to this, as a system accurately reflecting the designer's intention, attention is being paid to a CAD system based on constraints. That is, a system which calculates a geometric model using the relationship between basic solids, for instance, coincidence of planes or the distance between edges as an input. Such a relative relationship is called "geometric constraint." It can be said that specification by "structural constraint" such as parallelism or coincidence or "size constraint" such as inter-plane distance directly reflects the designer's intention.
For instance, in Japanese Published Unexamined Patent Application No. 02-186482, there is disclosed a method for generating a geometric model in which geometric constraints can easily be inputted by design commands. In the correction mode, variable values are substituted in the order of input. However, correction of the shape is allowed only by changing the values defined when inputted, and the constraint itself cannot be changed. In addition, since the order of the correction process is fixed by the order of input, a necessary and minimum modification process cannot be performed.
Also, in Japanese Published Unexamined Patent Application No. 62-272366, there is disclosed a graphic information processing system which indirectly holds geometric attribute values such as coordinate values using a pointer. By making geometric values common to a plurality of geometric elements, the attribute values of a plurality of geometric elements can simultaneously be modified in one correction process. However, if the order of tracing of the pointer is fixed, the extent covered by the modification process is limited, and a solution can be obtained as a shape. A correct result may not be achieved. Further, inconsistency in the value of size and the like cannot be handled.
Further, in Japanese Published Unexamined Patent Application No. 02-83782, there is disclosed a graphic information processing system wherein geometric constraint relations are generated by selection of basic solids and relative definition of geometric elements (such as inter-plane distance), and correction, modification and classification are performed on the basis of the geometric constraint relations. In this system, however, dependency relations are not fixed, and thus identification of the fixed planes which are not changed is not uniquely given. If the selection of the fixed planes is not proper, the system does no more than display the constraint relations that are not satisfied, and no measures are presented for resolving the inconsistency. In addition, since only plane information is treated, geometric specification of sides and vertexes cannot be done.